This invention relates to a method for providing thermal protection for re-entry vehicles. More particularly, this invention concerns itself with the implantation into the nose-tip of a re-entry vehicle of a carbon-carbon implant material having a different density, usually but not necessarily greater and greater roughness than the material of the main portion of the re-entry vehicles nose cone.
At the present time, a considerable research effort is being undertaken in an attempt to provide significant technical advances and improvements in re-entry vehicle technology. Of particular concern is the provision of improved nosetip material concepts in terms of higher performance and lower recession. Among the more important structural materials recently suggested are those of a carbonaceous content and of woven construction. This type of material is generally referred to as a carbon-carbon fibrous composite since, in its fully processed state, both the woven fibers and their binding resinous matrix are generally carbon or graphite. Such materials are well known for their use in re-entry vehicles and are exemplified by the teachings set forth, for example, in U.S. Pat. Nos. 4,016,322, 4,063,684 and 4,131,708 issued Apr. 5, 1977, Dec. 20, 1977 and Dec. 26, 1978 respectively.
As is well known, the construction characteristics of carbon-carbon materials influence the boundary layer transition progression on the nose-tip of reentry vehicles through the intrinsic surface roughness features that develop during ablation of the nose-tip during its re-entry into the earth's atmosphere. Subsequent to the onset of nose-tip boundary layer transition, these tips change shape from a blunt laminar configuration to a geometrically sharp turbulent configuration through a continuing ablative shaping process. Variabilities in transition onset altitudes and forward progression may occur as caused by variations in material properties that govern the material surface roughness. Such variations may also involve non-uniformities on the same tip which produce nosetip asymmetries. Such events influence vehicle performance since nosetip shaping asymmetries produce vehicle trims which affect the vehicle motion. A representative shape change sequence is depicted in FIG. 1 where viewing the shapes from left to right portrays the ablative responses during the reentry regime of a typical nose cone. A computed asymmetric shaping sequence is depicted in FIG. 2 in which case the asymmetry is caused to develop by an assumed non-uniformity in the nosetip's intrinsic roughness.
A number of technical advances have been made in the area of nose-cone technology, notably, as noted above, in the use of carbon-carbon composites as structural materials in their fabrication. However, in spite of these advances, variations in material surface roughness ultimately produce nose tip assymetries that, in turn, effect the flight characteristics of the re-entry vehicle. However, with the present invention, a technique has been developed that unexpectedly overcomes these anomalies by using a carbon-carbon nose cone implant in the leading surface of the tip of the nose cone. The carbon-carbon material implanted in the nose tip is characterized by having a usually greater density and a greater roughness than the carbon-carbon material forming the main portion of the nose cone. As a consequence, it has been found that when the implantation material is exposed to the boundary layer, its difference in surface roughness and density from that of the material of the nose cone itself, will uniformly trip the nosetip boundary layer causing the transition forward progression, and hence the sharpening of the nose cone to proceed symmetrically rather than asymmetrically as occurred heretofore.